The formation of the oxide ion, ${\mathrm{O}}^{2-}\left(\mathrm{g}\right),$ from oxygen atom requires first an exothermic and then an endothermic step as shown below:

$\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^{-}\left(\mathrm{g}\right) ;\mathrm{ }∆{\mathrm{H}}^{\ominus }=-141\mathrm{ }\mathrm{kJ}\mathrm{ }{\mathrm{mol}}^{-1}$

${\mathrm{O}}^{-}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^{2-}\left(\mathrm{g}\right) ; ∆{\mathrm{H}}^{\ominus }=+780 \mathrm{kJ}\mathrm{ }{\mathrm{mol}}^{-1}$

Thus process of formation of ${\mathrm{O}}^{2-}$ in gas phase is unfavourable even though ${\mathrm{O}}^{2-}$ is isoelectronic with neon. It is due to the fact that,